Refractory ceramics (e.g., aluminates, aluminosilicates) can exhibit several enhanced properties relative to refractory metals and alloys, such as corrosion resistance, high temperature stability in oxidizing atmospheres, specific strength and stiffness, creep resistance, and wear resistance. However, the brittleness of ceramic bodies renders the fabrication of shaped components tedious and expensive. Such brittleness also necessitates the use of reinforcements to enhance damage tolerance.
Despite extensive research and development over the past few decades, continuous-ceramic-fiber-reinforced, ceramic-matrix composites have not found widespread use in high-temperature structural applications involving oxidizing atmospheres (e.g., turbine engine applications). The use of these materials has been hampered by the inability to produce low-cost, net-shaped composites that are capable of retaining both a high strength (i.e., owing to a high fiber strength) and a high toughness (i.e., owing to sufficiently weak fiber-matrix interface for fiber-pullout toughening) under such high-temperature, oxidizing conditions (e.g., ≈1200xc2x0 C. in high-pressure air for turbine engine applications) [1-7]. Erosion/oxidation resistance are of particular concern in the very high-temperature (≈2000xc2x0 C.) corrosive conditions of liquid-fueled rocket engines [7]. Although continued research and development of high-strength oxide fibers and oxidation-resistant interface coatings may ultimately enable the low-cost fabrication of oxide fiber-reinforced ceramic composites with satisfactory damage tolerance and erosion/oxidation resistance for jet and rocket engine applications, oxide-matrix composites with other types of reinforcements should be considered in the interim.
An alternate approach for reinforcing oxide matrices is to use metallic (or intermetallic) alloys that are oxidation resistant, such as Ni-based compositions, or very high-melting, such as Nb-based compositions (Nb, Nbxe2x80x94Al solid solutions, and Nb3Al melt at 2468xc2x0 C., 2060-2468xc2x0 C., and 1940-2060xc2x0 C., respectively [8,9]). As reinforcement materials, Ni-rich or Nb-rich solid solutions, intermetallic compounds, or mixtures thereof offer a number of attractive features.
For example, owing to their oxidation resistance at elevated temperatures, damage tolerance (e.g., yield strength, ultimate tensile strength, toughness, fatigue resistance) over a range of temperatures, and capability for being fabricated into complex shapes (e.g., by casting), Ni-based superalloys are used extensively for high-temperature components in engine applications (e.g., combustor liners and ducts, disks and blades, and nozzle vanes in turbine engines; exhaust valves in reciprocating engines; pre-combustion chambers in diesel engines) [5-7,10-13]. Aluminum additions are used to enhance the high-temperature strength of such superalloys, owing to the formation of the ordered compound xcex3xe2x80x2-Ni3Al as a coherent precipitate dispersed within the Ni-rich (FCC xcex3 phase) solid solution matrix [1,5,11,14-19]. Pure xcex3xe2x80x2-Ni3Al and its alloys exhibit higher yield strengths at elevated temperatures than the xcex3 phase [11,14-19]. The xcex3xe2x80x2 compound can also act as a ductile reinforcement material in composites (at room temperature or elevated temperatures), either as a polycrystalline material with proper alloying (e.g., Al concentration  less than 25 atom percent(at%) with dopants such as boron and chromium) or in single crystal form (e.g., as single crystal filaments) [11,14-18]. Stoichiometric xcex2-NiAl, while less ductile at room temperature than Ni-based superalloys and xcex3xe2x80x2-Ni3Al alloys, possesses a higher elastic modulus, a higher thermal conductivity, and a higher melting point (1638xc2x0 C.) [15,19-23]. Niobium alloys and internetallic compounds (e.g., Nb3Al) are very high melting and exhibit better creep resistance than Ni3Al, possess high thermal conductivities at elevated temperatures, and are less dense than other refractory metals (such as W, Ta, Mo) [8,24-26].
As indicated by the discussion above, these Ni-bearing and Nb-bearing alloys offer a tailorable range of thermo-mechanical properties that can be exploited in ceramic composites. In addition, the negative features of these alloys (i.e., weight, creep) can be significantly reduced if such alloys are used as reinforcements within composites containing a stiff, continuous, light-weight ceramic phase.
Accordingly, there remains a need for a method for the fabrication of near net-shaped AEAl2O4-bearing (where AE=Mg, Ba) composites reinforced with M-Al (M=Ni, Nb) solid solutions and/or intermetallic compounds (e.g., Nb, NiAl+Ni3Al, or Ni3Al+Ni-based solid solutions) [27].
Because the ionic species in these aluminates have essentially fixed valence states, these oxide compounds are thermodynamically compatible with oxygen at high temperatures. On the other hand, Nb, Ni, (Nb,Al) or (Ni,Al) solid solutions, and xcex3xe2x80x2-Ni3Al can act as ductile reinforcements at ambient and elevated temperatures (xcex2-NiAl also becomes ductile above xcx9c400xc2x0 C.) [15,17,19,23]. The thermal conductivities of Nb, Ni, and Nixe2x80x94Al alloys (particularly xcex2-NiAl) are much higher than for the aluminates [25]. The coefficients of thermal expansion (CTE) of the aluminates are smaller than for nickel or the nickel aluminides, which should place the ceramic phase in compression upon cooling from the peak processing or use temperature [9,32]. For very high-temperature applications, composites of AE aluminate and Nb or Nb3Al are attractive in that these phases possess similar CTE values (8.2-9.2X10xe2x88x926/xc2x0C. [9,32]).
Although the properties of AEAl2O4/Mxe2x80x94Al alloy composites will depend on factors such as the amounts, sizes, and distributions of phases, and the degree of interfacial bonding, the discussion above (and in the following sections) indicates that such composites can be:
lighter and more resistant to high-temperature creep, oxidation, and erosion relative to monolithic metallic alloys or intermetallics,
more fracture resistant (tougher, higher fracture strengths) and thermally conductive than monolithic aluminates,
formed into near net shapes by the process of the present invention as described herein.
Accordingly, and with the foregoing objectives and advantages in mind, the present invention is summarized below. In view of the present disclosure or through practice of the present invention, other advantages may become apparent.
A variety of reaction-based techniques have been developed for the in-situ syntheses of monolithic ceramics and ceramic composites, including the Co-Continuous Ceramic Composite (C4) process [1,2], the reactive metal penetration process (RMP) [3-5], the Directed Metal Oxidation (DIMOX) [6,7], the Reaction Bonded Metal Oxide (RBMO) process [8,9], Self-Propagating High-Temperature Syntheses (SHS) [10], Hillig or Hucke Process [11-13], and the Solid Metal-Bearing Precursor (SMP) process [14-18].
Such AEAl2O4/Mxe2x80x94Al alloy composites should exhibit enhanced thermo-mechanical properties relative to monolithic AEAl2O4 or monolithic Mxe2x80x94Al alloy bodies. Compared to Nb, Ni, and Nixe2x80x94Al alloys, polycrystalline MgAl2O4 and BaAl2O4 possess higher values of elastic moduli, lower densities, and, hence, higher values of specific stiffness (e.g., E/xcfx81=79, 40, 24, 23, and 12 MPaxc2x7cm3/g for MgAl2O4, NiAl, Ni3Al, Ni, and Nb respectively, at room temperature) [11,15,22,23,28]. These stiff, stoichiometric aluminates exhibit better creep resistance at elevated temperatures than Nb, Ni, or Nixe2x80x94Alxe2x80x94based solid solutions or intermetallic compounds [29-31].
The DCP process described in the present disclosure differs from several of these approaches, in that it does not rely upon oxidation reaction(s) involving externally-supplied, gaseous oxidants (e.g., the DIMOX, RBMO, SHS, and SMP processes involve oxidation reactions with externally-supplied O2(g)). The fabrication of large parts by processes that involve the use of a fluid, external reactant (DIMOX, RBMO, SHS, SMP) can require relative long oxidation times, as the reactant must migrate inwards through the thickness of the precursor from the outside surfaces.
Since the reactions involved in the DCP process do not involve an externally-supplied gaseous reactant, the rate of conversion does not scale with the size of the starting preform. The DCP and Hillig/Hucke processes both involve the reaction of a porous, solid preform with a liquid that has been infiltrated into the preform. However, unlike the Hillig/Hucke process, the porous, solid preform in the DCP route undergoes a displacement reaction with the liquid phase. The C4 and RMP processes also involve a displacement reaction between a fluid metallic phase and a solid oxidant. However, an important distinction between the DCP process and the C4 and RMP methods is the difference in volume of the solid products and reactants. In the C4 and RMP processes, a liquid metal reacts with a solid oxidant to produce a ceramic-metal composite, such as by the following net reaction:
4Al(l)+3SiO2=2Al2O3+3(Si)xe2x80x83xe2x80x83(1)
where (Si) refers to silicon dissolved in molten aluminum. The volume of 2 moles of alumina is less than the volume of 3 moles of silica. The open space formed as a result of this reduction in solid oxide volume is filled by molten metal (Alxe2x80x94Si alloy) that penetrates into the solid preform during reaction. Because the molten metal accommodates this change in volume, near net-shaped parts can be produced by the C4 and RMP processes after solidification of the molten claim. However, due to the reaction-induced reduction in solid volume, the ceramic content of the final shaped part produced by the C4 and RMP processes will be less than that of the preform. The reaction(s) involved in the DCP process result in an increase in solid volume, so that ceramic content of the final shaped part is greater than that of a porous preform. Hence, shaped parts containing relatively high fractions of ceramic phase can be produced by the DCP process.
In accordance with the present invention, the following general displacement reaction between a liquid species, M(l), and a solid preform comprising the compound, NBXC(s):
AM(l)+NBXC(s)=AMXC/A(s)+BN(l/g)xe2x80x83xe2x80x83(2)
where MXC/A(s) is a solid reaction product (X is a metalloid element, such as, for example, oxygen, nitrogen, sulfur, etc.) and N(l/g) is a fluid (liquid or gas) reaction product. A, B and C are molar coefficients.
In the DCP process of the present invention, the reactants and reaction conditions are chosen such that the volume of xe2x80x9cAxe2x80x9d moles of the solid product, MXC/A(s), is greater than the volume of one mole of the solid reactant, NBXC(s). If the NBXC(S) preform is porous, then this reaction-induced volume increase can be accommodated by such porosity, so that the external dimensions of the final shaped, MXC/A(s)-bearing part can be close to those of the porous preform. Reactions of the type (2) that involve an increase in solid volume are a key feature of the DCP process of the present invention. Such volume-increasing, liquid/solid displacement reactions have not been used to date to produce near net-shaped, dense ceramic bodies.
Accordingly, the methods of the present invention include a method for producing a material selected from the group consisting of ceramics and ceramic composites, the method comprising reacting: (1) a fluid comprising at least one displacing metal; and (2) a rigid, porous ionic material having a pore volume and comprising at least one ion (which for instance and preferably may be derived from a non-alkaline earth metal), the at least one displacing metal capable of displacing the ion(s); and allowing the fluid to infiltrate the ionic material or compound such that the at least one displacing metal at least partially replaces (and preferably substantially or completely replaces) ion(s), such as non-alkaline earth ion(s), and so as to at least partially fill (and preferably substantially or completely fill) the pore volume, and so as to produce a ceramic or ceramic composite. Whether the resultant material is a ceramic or ceramic composite will depend upon whether the displaced metal remains in the product material to form another phase. If so, the resultant product will be a composite; if not, the resultant product will be a ceramic, only containing a ceramic phase.
As used herein, xe2x80x9coxidationxe2x80x9d refers to the process by which metal elements are converted to a higher valence state (i.e., metal cations), and xe2x80x9creductionxe2x80x9d refers to the process by which metal cations are converted to a lower valence state (i.e., metal elements). A xe2x80x9cdisplacement reactionxe2x80x9d shall refer to a process in which at least one first displacing metal cation undergoes reduction as a result of a reaction with at least one second metal element whichundergoes oxidation. For the purposes of this specification, the alkaline earth metals are berylium, magnesium, calcium, strontium, barium and radium.
The displacing metal(s) may be any metal(s) adapted to replace the ion(s) of the ionic material or compound, and for instance may comprise alkaline earth metal(s) which, for instance, may be selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof. For instance, the displacing metal(s) may be selected from the group consisting of non-alkaline earth ions, such as ions derived from metals such as aluminum, nickel and niobium.
The rigid, porous ionic material(s) may comprise any ionic material or compound(s) adapted to contain the ions to be displaced, such as ions derived from non-alkaline earth metal(s), for instance those materials that may be selected from the group consisting of oxides, sulfides, nitrides and halides. The rigid, porous ionic material(s) also may comprise ionic materials or compounds selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, and niobates. The non-alkaline earth metal(s), for instance, may be selected from the group consisting of aluminum, nickel, niobium.
To make a net shaped article by the method of the present invention, the rigid, porous ionic material is preformed into a shape, and, following the process of the present invention, the ceramic or ceramic composite maintains that shape, typically within a few percent of the original shape. This preforming may be done by a light sintering, for instance.
In a preferred embodiment, the fluid is a liquid, the liquid being supplied by a melting solid comprising the displacing metal(s), such one or more alkaline earth metals. It is preferred that the two solid reactive components (1) and (2) be placed in contact while in a solid state for ease of handling, then allowing the melting solid (i.e., a solid that would become melted at or before reaching the processing temperature) to melt so as to bring the liquid phase into contact with the rigid, porous ionic material. The same may also be accomplished by using a solid material that may sublime to produce such a fluid as reactant component (1) above.
The method of the present invention may also be described as a method for producing a ceramic or ceramic composite, the method comprising reacting: (1) a fluid comprising at least one displacing metal; and (2) a rigid, porous ionic material having a pore volume and comprising at least one ion, such as a non-alkaline earth ion, the at least one displacing metal capable of displacing the at least one ion; and allowing the fluid to infiltrate the ionic material such that the at least one displacing metal at least partially replaces (and preferably substantially and even completely replaces) the at least one non-alkaline earth ion, and so as to at least partially fill (and preferably substantially and even completely fill) the pore volume and so as to undergo a general displacement reaction between reactants comprising a liquid species M(l) derived from the fluid, and the rigid, porous ionic material of the general formula, NBXC(s), as follows:
AM(l)+NBXC(s)=AMXC/A(s)+BN(l/g)
wherein MXC/A(s) is a solid reaction product and wherein X is a metalloid (i.e., a metal or metalloid element), N(l/g) is a fluid reaction product, and A, B and C are molar coefficients; and wherein the reactants are chosen such that the volume of A moles of the solid reaction product MXC/A(s) is greater than the volume of one mole of the solid reactant, NBXC(s), such that the reaction-induced volume increase can be accommodated by such pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.
The displacing metal(s), ion(s), and rigid, porous ionic material(s), as well as other preferred aspects of the method of the present invention, may be as described and exemplified above.
The method of the present invention may also be described as a method for producing a ceramic or ceramic composite, the method comprising the steps: (a) placing in contact:
(1) a solid adapted to produce a fluid material comprising at least one displacing metal;
(2) a rigid, porous ionic material having a pore volume and comprising at least one ion, such as those derived from non-alkaline earth metal(s), maintaining the solid at sufficient temperature such that the solid produces the fluid, the fluid infiltrating the ionic material so as to at least partially replace (and preferably substantially and even completely replace) the ion(s), and so as to at least partially fill (and preferably substantially and even completely fill) the pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.
The displacing metal(s), ion(s), and rigid, porous ionic material(s), as well as other preferred aspects of the method of the present invention, may be as described and exemplified above.
The present invention includes any ceramic or ceramic composite prepared in accordance with the inventive methods described herein.
The DCP process of the present invention comprises three basic steps:
1) Synthesis or other acquisition of a porous preform: A porous preform with an appropriate composition, pore fraction, and overall shape is prepared or obtained. The pore fraction of the preform is tailored so that the reaction-induced increase in solid volume can compensate partially or completely for such porosity. It will be understood that the porous preform need only be sufficiently dimensionally stable to resist the capillary action of the infiltrated liquid reactant.
2) Infiltration: The porous preform is infiltrated with a fluid (liquid or gas) reactant (which, if liquid, may come from the melting of a solid or otherwise from a substance introduced to the reaction in liquid form.
3) Reaction: The fluid reactant is allowed to react partially or completely with the solid preform to produce a dense, shaped body containing desired ceramic phase(s).
As the DCP process of the present invention involves the reaction of liquid and solid phases throughout the infiltrated preform, the time required to form the desired ceramic phase(s) should depend on interfacial areas and/or the sizes of the reacting phases, and should not depend on the overall dimensions of the preform. As a result, the reaction time required for large parts can be relatively modest. By properly tailoring the pore fraction and composition of the preform, the shape and dimensions of the final transformed part can be close to those of the porous preform. Hence, costly ceramic machining of the final part can be minimized or avoided. Further, since porous ceramic preforms of desired shape can be produced by relatively low cost processes (e.g., slip casting, cold pressing), the DCP process is an inexpensive means of fabricating dense and shaped ceramic-bearing bodies.
By tailoring the composition of the liquid and the preform, the DCP process can be used to fabricate near net-shaped composites with a wide range of ceramic and metal contents. For example, the present invention may be used to synthesize co-continuous composites of refractory, alkaline-earth aluminates with high-melting metallic or intermetallic reinforcements (e.g., MgAl2O4/Nb, BaAl2O4/Ni3Al+NiAl composites). Such AEAl2O4/Mxe2x80x94Al composites should exhibit enhanced stiffness/creep resistance, improved erosion/oxidation resistance, and lower densities (xcfx81MgAl2O4 less than 0.5xcfx81Ni or 0.5xcfx81Nb) than pure metals or intermetallics. These composites should also possess higher values of fracture strength, toughness, and thermal conductivity than monolithic aluminates.
The method of the present invention may thus be carried out by the fabrication of a shaped, porous preform of solid oxide+metallic (or intermetallic) reactants, followed by the (preferably) pressureless infiltration of the preform with a low-melting, alkaline-earth-bearing liquid, followed by, or accompanied by, heat treatment to allow for an internal displacement reaction that converts the liquid/solid mixture into a dense, near net-shaped, high-melting ceramic/metal alloy composite.
The present invention also includes the ceramic/metal alloy composites prepared by the method of the present invention.
The present invention may be understood by reference to the processing/microstructure/property correlations for DCP-derived, AEAl2O4/Mxe2x80x94Al alloy composites (AE=Mg or Ba; M=Ni or Nb; Mxe2x80x94Al=metallic alloy and/or intermetallic compound) described herein. In accordance with the present invention, one skilled in the art may make additional adjustments to the processing parameters to affect the macrostructure (shape, dimensional retention) and microstructure (phase fraction, size, chemistry; interface chemistry/morphology) of composites of the present invention.
The microstructural features of ceramic composites of the present invention are believed to improve mechanical properties (e.g., MOR, toughness/damage tolerance, and creep). Through the present invention, one may tailor the microstructure of ceramic composites without compromising the shape retention capability.
AEAl2O4/Mxe2x80x94Al alloy composites of the present invention may be fabricated by infiltrating AE-bearing liquids into porous, oxide+metal preforms, and then conducting a displacement reaction of the following type:
{AE+wAl+xM}(l)+MyAlz(s)+MAl2O4(s)xe2x86x92AEAl2O4(s)+M1+x+yAlw+z(s)
where AE is one or more alkaline earth metal, preferably Mg or Ba or Ca; M is a non-alkaline earth metal, such as Ni or Nb; bracketing xe2x80x9c{ }xe2x80x9d refers to species dissolved in a liquid; MyAlz(s), M1+x+yAlw+z refer to solid solutions, compounds, or mixtures of both. Such composites may include, for example, i) MgAl2O4 or BaAl2O4 as the matrix phase, and ii) Nb or Nixe2x80x94Al metallic solid solutions and/or intermetallic compounds (e.g., Ni3Al, NiAl) as the reinforcement phase(s).
Dense, shaped, aluminate-matrix composites with varied amounts of Mxe2x80x94Al reinforcements may be produced by tailoring the composition of the liquid (AE, w, and x), and the pore fraction (P) and phase content (M, y, and z) of the preform. For example, a composite of 90.5 vol % BaAl2O4 and 9.5 vol % Ni0.94Al0.06 solid solution can be produced if w=0.06, x=y=zxe2x88x920, and P=0.43. However, if w=0.95, y=6.00, x=z=0, and P=0.32, then a composite of 54 vol % BaAl2O4 and 46 vol % Ni0.88Al0.12 solid solution can be synthesized.
Ceramic and reinforcement phase sizes in the transformed composites may be adjusted by varying the sizes of pores and reactant phases in the preform (i.e., by controlling particle sizes/distributions and compaction/sintering conditions).
Dopants optionally may be introduced into the liquid or the preform to alter the interface chemistry and morphology in the reacted composites.
By altering mechanical behavior through changes in microstructure/microchemistry, and tailoring the flexible DCP process of the present invention to obtain particular microstructural/microchemical features, one of ordinary skill may produce near net-shaped ceramic-matrix composites (e.g., for rocket or jet engine components), in accordance with desired enhanced performance.
The feasibility of fabricating shaped ceramic composites by the DCP process may be demonstrated by the syntheses of MgO/Mgxe2x80x94Al composites from porous Al2O3 preforms, as per the following reaction:
3Mg(l)+Al2O3(s)=3MgO(s)+2(Al)xe2x80x83xe2x80x83(3)
where (Al) refers to aluminum dissolved in a magnesium-rich melt. Three moles of MgO possess a volume that is 32% greater than one mole of Al2O3, so that this reaction is of the type (2), with A=3, B=2, C=3, M=Mg, N=Al, and X=O. Disk-shaped Al2O3 preforms (10 mm dia., 2 mm thick) with 29% porosity were prepared by pressing and partial sintering (for 8 h at 1500xc2x0 C.) of Al2O3 powder. The disks were sealed within a steel can along with solid pieces of magnesium. The sealed can was then placed in a reaction furnace at 1000xc2x0 C. for 15 h. Under these conditions, the magnesium melted, infiltrated, and reacted with the porous alumina preforms to yield 98% dense composites of 89.7 vol % MgO and 10.3 vol % Mgxe2x80x94Al alloy. The composites retained the shape and dimensions (to within 2.3%) of the porous alumina preforms. Hence, the porosity within the alumina preform was substantially compensated by the reaction-induced increase in solid volume.
The DCP method of the present invention has been used to produce near net-shaped MgO and MgAl2O4-bearing composites reinforced with lightweight Mgxe2x80x94Al or higher-melting (and stronger) Fexe2x80x94Nixe2x80x94Al alloys. Composites reinforced with the latter alloys possessed fracture strength and toughness values up to 470 MPa and 13 MPxc2x7amxc2xd, respectively.
Several process modifications can be introduced to the basic process discussed above to produce reinforced ceramic composites:
1). Instead of starting with a pure elemental reactant M(l) in reaction (2), an alloy liquid containing species M can be used. The other elements in the starting liquid alloy may be inert (and thereby wind up as additional metallic phases in the final part), may react with the ceramic preform (to yield other oxidized phases), or may alloy with or react with the species N liberated by reaction (2) (to yield a compound containing N). For example, if Mg is alloyed with Ni, then reaction (3) may be modified as follows:
3(MgNi2/3)+Al2O3(s)=3MgO(s)+2NiAl(s)xe2x80x83xe2x80x83(4)
where (MgNi2/3) refers to a molten Mgxe2x80x94Ni alloy, and NiAl(s) is an intermetallic compound.
2). By limiting the amount of one of the reactants, reaction (2) may also not be allowed to go to completion. This would allow for the formation of a wider range of ceramic compositions in the final part. For example, if the ratio of magnesium to alumina in reaction (3) were reduced from 3:1 to 3:4, then the following reaction can occur:
xe2x80x833Mg(l)+4Al2O3(s)=3MgAl2O4(s)+2(Al)xe2x80x83xe2x80x83(5)
(Note: 3 moles of spinel, MgAl2O4, possess a larger volume than 4 moles of Al2O3, so that this reaction is still of the type (2).)
3). A gaseous reactant species (M in reaction (2)) could be used instead of a liquid species. For example:
3Mg(g)+4Al2O3(s)=3MgAl2O4(s)+2(Al)xe2x80x83xe2x80x83(6)
4). Additional phases may be added to the porous preform. Such phases may act as reinforcements in the final component. For example, SiC could be added to an alumina preform to produce MgO/SiC/(Al) composites via reaction (3).
It is important to note that, although reactions (3)-(6) involve the consumption and formation of oxide materials or compounds, the DCP process also may be applied to the fabrication of other ionic materials or compounds (e.g., nitrides, sulfides, etc.). The DCP process refers to the use of an in-situ displacement reaction involving an increase in solid volume to produce a ceramic-bearing body with a lower pore fraction than the starting preform (i.e., due to compensation of at least some of the porosity in a porous preform by the displacement reaction).